1. Field of the Invention
The present invention pertains to a method of reducing metal oxides to metals using an electrochemical reduction device in which LiCl-based salt is employed as an electrolyte. In particular, it can be used to reduce spent oxide nuclear fuel to nuclear-fuel metal utilizing the reduction device with LiCl—Li2O salt.
2. Description of the Prior Art
As well known to those who have experience in the art, an electrochemical reduction device consists, in general, of a cathode, an anode, and an electrolyte. Chemical changes such as reduction and decomposition of compounds are occurred in these kinds of apparatuses by difference in electric potentials. Many instruments have been devised especially in recovering highly purified metals from metal oxides.
Argonne National Laboratory (ANL) in the USA has developed a method of recovering nuclear-fuel metal from spent oxide nuclear fuel using an electrochemical reduction device in which the spent oxide nuclear fuel is reduced to the nuclear-fuel metal in molten LiCl-based salt at high temperature. Actinide elements in the spent oxide fuel are recycled according to the IFR (integral fast reactor) nuclear fuel cycle process. Researchers of the ANL have developed a fuel recycle technology to reuse the spent oxide nuclear fuel by reducing into metal in the molten LiCl-based salt at high temperature and found that a LiCl—Li salt system is sufficiently competitive in recovering the spent oxide fuel to the corresponding metallic form considering the reaction temperature and corrosiveness of materials.
The conversion efficiency of the reduction reaction increases as Li metal reacts with the spent oxide nuclear fuel into the nuclear-fuel metal and Li2O which is dissolved in molten LiCl-based salt then recycled to the reactor by Li electrowinning in LiCl—Li2O salt.
On the other hand, the method has the following disadvantages:
First, when the concentration of Li2O produced during the reduction of the spent oxide nuclear fuel increases, the conversion efficiency of some elements in the spent oxide nuclear fuel such as uranium and plutonium oxides decreases and undesirable side products such as complex oxides and chlorides are produced since Li2O with rare-earth elements. Accordingly, it takes a relatively long time of 80 hours to convert the spent oxide nuclear fuel effectively to the nuclear-fuel metal and the conversion efficiency of the spent oxide nuclear fuel reaches an unsatisfactory value while maintaining the concentration of Li2O below 3.2 wt %.
Second, both the inside and the outside of the reactor must be kept under an inert atmosphere since the Li metal is vulnerable even to small amount of oxygen and moisture.
Third, there is difficulty to form the LiCl—Li salt system because of the low solubility of the Li metal to the molten LiCl and the density difference between molten LiCl and Li metal. Therefore, a mixture of the LiCl-based liquid salt and the Li metal must be agitated mechanically for the uniform dispersion of the floating Li and the consequent enhancement of the reaction rate as disclosed in Japanese Pat. No. 2000-292593.
Japanese Pat. No. 2000-131489 describes the simultaneous performance of the first process of reducing the spent oxide nuclear fuel to the nuclear-fuel metal and the second process of recycling the Li metal into the first stage. However, the patent discloses that the first and the second process are independently conducted when the first process is combined with the second process. Therefore, the patent fails to clear up the listed disadvantages, and is unfavorable since it is difficult to connect the first and the second process for subsequent processes because of complexity of whole processes including the above processes. Furthermore, the patent is not competitive in terms of operational stability.
Accordingly, there remains a need to develop a method capable of overcoming the above disadvantages.